A circuit comprising a plurality of units with semiconducting elements often constitutes an integral part of an electric power converter, where they are used as power electronic switches. These switches are connectedly arranged in series, where each switch is capable of maintaining a part of the voltage applied over the converter. Known power semiconductors are capable of holding a voltage of 1 to 6 kV. By series connection of a plurality of such switches a converter may maintain a voltage within a range of 10 to 500 kV. Each switch comprises a plurality of semiconducting elements that may be connected in series and/or in parallel to achieve a performance of desire. The series connection will increase the voltage maintenance and the parallel connection will increase the current capacity.
In a voltage source converter (VSC) the electronic power switches comprises semiconductors of the turn-off type. Such converters are often used in high voltage direct current (HVDC) applications for converting direct current to alternating current or conversely. Such converters are also used in static var compensators (SVC) and reactive power compensation (RPC) plants for balancing the power transmission within a power network.
Semiconductors like GTO thyristors and IGBT are suitable for high power applications. Semiconductors of the latter kind is often preferable since they combine good power handling ability with properties which make them well suited for connection in series. They may be turned off with high accuracy. In such constellations a plurality of IGBTs form valves in a voltage source converter for handling voltages up to 500 kV.
Short circuits situations may occur in semiconductor circuits. In such a situation it is necessary to be able to handle the effect of the short circuit. When a semiconductor breaks down as a result of an overcurrent or overvoltage the semiconductor cannot hold a voltage any longer. A damaged semiconductor cannot be controlled. It may hold only a small voltage difference and when conducting there will be a small resistance. A less pleasant performance is the heat generation. Forcing a current through a damaged semiconductor will generate an arc with a voltage drop of approximately 10-20 V that will generate extensive power dissipation. This may either develop into a melt-down of the component or into a fire that may destroy the whole valve.
A converter valve comprises a plurality of semiconducting valve units connected in series. Each of these valve units is designed to handle a determined part of the overall voltage of the valve and to transfer the total current of the valve. Each valve units comprises a plurality of semiconducting elements connected in parallel. Each parallel connected semiconducting element is thus designed to transfer a part of the total current through the valve unit. Now, if one of these semiconducting elements fails, that valve unit will no longer be capable of holding a voltage difference. Still when the whole valve is controlled into a closed circuit, a part of the current or the total current will pass the faulty semiconductor and thus develop heat.
Ta avoid such a situation the semiconducting element used today comprises a special feature of assuming a closed circuit after a fatal breakdown has occurred. By assuming a closed circuit no heat will be generated in the faulty semiconductor. Thus, in the situation described the semiconducting elements in one valve unit are still capable of transferring the same current as would have been when all semiconducting element were in operation. Hence, when one of the semiconductors in a valve unit fails the other semiconductors of that unit are controlled to assume a steady closed circuit. This will result in the unit no longer being capable of holding a voltage but still conduct the current without heat generation.
From a voltage aspect, however, the failing unit will not withstand any voltage since at least one semiconducting unit is always short circuited. This has the effect that the voltage applied over the valve which normally is split up by a plurality of switching units now has to be split by the same number but one. Since the number of series connected units are typically in the range of 100 to 500 the voltage overload is in the range of 0.2 to 10%. This is fully within the voltage overload capacity of the semiconducting element.
The technique of using semiconducting elements which are specially designed for these situation functions very well. These semiconducting elements are, however, costly to produce. Hence, there is a need to lower the cost of the converter but still achieve the same performance.